1. Field of the Invention
This invention pertains to electrically controlled actuators and more particularly pertains to microelectromechanical systems (MEMS) actuators.
2. Description of the Related Art
Electrically controlled actuators receive electrical signal input and provide a mechanical output that can be used to move objects. The electrical signal can control the mechanical output to control a rate and amount of movement applied to the object. Large, electrically controlled actuators are common in mechanical systems to control valves, pumps, and move objects.
Recent innovations require control of very small components. In U.S. Pat. No. 5,808,384 a photolithographic process is used to fabricate a micromechanical actuator to control switches, relays, and valves. However, the invention of the U.S. Pat. No. 5,808,384 does not teach a means to move objects toward and away from the plane of the substrate upon which the device is formed.
In U.S. Pat. No. 5,867,297 the inventor discloses a microelectrical mechanical device to oscillate a mirror for deflecting a laser beam to read bar codes. The mirror is substantially upright and moved about a hinge connection to the substrate on which it is formed, but the device relies upon staples and hinges, which have proven unreliable. In addition, the mirror is hinged to the substrate and is constrained to move in rotation about a single axis, thereby limiting its utility. Also, some applications require a mirror that is parallel to the substrate on which is formed, a design imitation that can not be accommodated by U.S. Pat. No. 5,867,297.
Currently, many applications seek to control a mirror surface to control light energy and move light beams in order to scan bar codes or create and project video images.
In U.S. Pat. No. 5,192,946 a spatial light modulator includes an array of mirrors that are used to create and project a video image onto a screen. The video image is created by directing light onto the mirror array (referred to in U.S. Pat. No. 5,192,946 as a deformable mirror device) and controlling an orientation of the individual mirrors in order to control a path of the light. The screen image comprises many pixel elements that correspond to individual mirrors in the array. Each pixel element is turned on and off by controlling the orientation of the mirrors to reflect the light onto the screen (or intermediate lenses) or away from the screen. The mirror array includes mirrors formed on a semiconductor substrate that are hinged to move between up and down positions in which the light is reflected toward or away from the screen in order to toggle on and off, respectively, the pixel element displayed on the screen. The individual hinged mirrors can only be rotated about one axis, providing only one degree of motion that limits projector design. Such video image systems are also referred to as digital light processors (DLP).
In order for such video systems to be viable, the mirror arrays must be easy and inexpensive to fabricate in quantity and provide controllable, high speed movement of the mirrors. Another important criteria is that the mirror actuation means be very reliable. In many designs, semiconductor hinges have proven to have lower reliability than desired mainly because of stiction and abrasion problems. In addition, it is desirable that the mirrors of the mirror array can be moved a sufficient range of motion and freedom of movement to accommodate projector designs. Accordingly, where other factors are equal, actuators that can move mirror in 2 or 3 degrees of motion and with a range of motion of ±30 degrees about an axis of rotation is preferred to an actuator system that can move a mirror a maximum of ±10 degrees in only one degree of motion.